This invention relates to potentiometer assemblies and, more particularly, to potentiometer assemblies of the type suitable for use in deriving tuning voltages in a varactor tuned television receiver.
Until relatively recently, channel tuning in television receivers has been accomplished by mechanical mechanisms which were operated to physically switch tuning elements in the appropriate tuning circuits. Although tuners of this type have provided excellent performance through the years, due to their inherent mechanical nature they have characteristically exhibited various long-recognized deficiencies.
Electronic tuning systems overcome many of the problems heretofore associated with conventional mechanical tuners. The practicality of such electronic tuners has been made possible by the varactor diode, an electronic component which exhibits an output capacitance determined by the DC voltage impressed across its terminals. Thus, television tuning may be enabled by utilizing a potentiometer assembly to appropriately vary the DC voltages applied to a series of varactor diode tuning elements.
Potentiometer assemblies suitable for use in electronic tuning applications are frequently classified as either single turn devices or multiple turn devices. Single turn potentiometers are normally circular in configuration and include an element capable of carrying a contact through a single revolution along a resistive member. Conventionally, single-turn circular type potentiometers are most frequently found in VHF applications. Multiple-turn potentiometers are normally elongated linear type structures configured for receiving a threaded shaft which is operable for driving a contact carrying carriage member along a resistive element. Although not necessarily so limited, potentiometer assemblies of the latter type are particularly suitable to UHF applications where a rather large frequency band must often be entirely covered by the manipulation of a single potentiometer.
Prior art multiple-turn elongated type potentiometer assemblies have generally taken the form of individual rectangular housings configured for permanently and non-replaceably retaining its associated threaded shaft and carriage member. Consequently, if, during testing of the potentiometer assembly itself, or a finished tuner address assembly or sub-assembly, a defect is discovered in a potentiometer, the applicable maintainance action is to discard the entire potentiometer assembly in favor of a new unit. This is, of course, an economically wasteful procedure especially where the defect is attributable only to a minor component of the entire assembly. Moreover, in television tuning applications, a multiplicity of similar potentiometers are required, thus adding to the difficulty.
Futher, prior art multiple-turn potentiometer assemblies are prone to various deficiencies in their stability characteristics. Particularly in electronic television tuners where a single potentiometer is used to cover the entire UHF frequency spectrum the potentiometer carriage member be positionable with relative precision to produce a finely controllable tuning voltage for application to the tuning elements. Any appreciable change in resistance at the set position of the contact member may result in a detuning of the receiver. Extremely stable positional characteristics are also required in applications where a ceramic substrate or the like characterized by a patterned resistive coating is utilized in association with the potentiometer assembly to provide the requisite resistve member. In such cases, any misalignment or drift of the potentiometer assembly contact member relative to the substrate can result in a change in contact resistance which may detrimentally effect the performance of the tuning system.
Finally, it will be appreciated that the carriage member of a multiple-turn potentiometer assembly of the type described above will be limited in its extent of travel at either end of the potentiometer housing. Accordingly, it would be highly desirable to provide a mechanism to protect the unit from damage should, through inadvertence or otherwise, the threaded shaft be operated to attempt to drive the carriage member beyond its physical limit of travel in either direction.